GI Infections - Norovirus Flashcards
What are the features of Norovirus?
Family: Caliciviridae
Genus: Norovirus
Spherical nonenveloped 27nm virus
Positive-sense SS RNA
Subdivided into genotypes and then strains
5 genogroups: groups I, II and IV affect humans
What are the sources of Norovirus?
Environment: human intestinal tract
Foods:
direct hand-to-food contact during meal prep
shellfish harvested from waters contaminated with sewage
Waterborne
Describe the environmental stability of Norovirus
Survives: <10 ppm chlorine, freezing, and heating to 60°C
can persist in environment for up to 3-4 weeks
Where do outbreaks of Norovirus most commonly occur?
Outbreaks of norovirus infection often occur in closed or semiclosed communities, such as long-term care facilities, overnight camps, hospitals, prisons, dormitories, and cruise ships
What is the infective dose of Norovirus?
10 viral particles
Describe the pathogenesis of Norovirus
Noroviruses are transmitted person to person via direct contact, exposure to aerosols, or fecal–oral routes
Noroviruses bind polymorphic histoblood group antigens (HBGAs) that putatively serve as receptors or cofactors for infection.
Infection is characterized by damage to the microvilli in the small intestine. Upon microscopic investigation, villi are found to be blunted, although the mucosa and epithelium remain intact.
Noroviruses do not invade the colon, so fecal leukocytes are typically absent, and hematochezia is rare
What is the incubation period for Norovirus?
Most 24 to 48 h but can occur in 12 h
What are the symptoms of Norovirus?
Nausea
Vomiting (<50% of cases)
Diarrhea
Stomach cramping
Some patients: Low-grade fever Chills Headache Muscle aches A general sense of tiredness
How long does Norovirus last?
12 - 60 hours
What histological features do biopsies from Norovirus patients show?
Blunting and atrophy of the villi
Crypt hyperplasia
Mucosa and epithelium remain intact but there is an increase in epithelial cell apoptosis and damage to tight junction proteins
Why does diarrhoea occur in patients with Norovirus?
Diarrhea is induced because:
- decreased levels of some brush border enzymes
- carbohydrate and fat malabsorption
- leak flux mechanism: chloride secretion
Why does vomiting occur in patients with Norovirus?
Vomiting is related to virus-mediated changes in gastric motility and delayed gastric emptying
Can patients be reinfected with Norovirus?
Yes - no long-term immunity so reinfection can occur
- multiple genetic and antigenic types
- new variants through antigenic drift (sometimes recombination)
Who is susceptible to Norovirus?
Anyone
- can be more severe in elderly or immunocompromised or those with cardiovascular disease
What complications can occur in patients with Norovirus?
Renal failure
Arrhythmia
Hypokalemia
Chronic diarrhea
What laboratory methods can be used to diagnose Norovirus infection?
Electron microscopy
- Sensitivity: 106-107/ml of virus in stool
Immune electron microscopy (IEM)
Enzyme immunoassay detection using a commercial kit
Reverse transcription-polymerase chain reaction (RT-PCR) and nucleic acid hybridization assays
- high sensitivity: detect 102-104 viral particles/ml in stool
How is immune electron microscopy (IEM) performed to diagnose Norovirus?
Coat the examination grid of the microscope with convalescent-phase serum from patients before stool specimens are applied
- Ab in serum traps virus
- can improve the sensitivity of EM by 10 to 100 fold
When using enzyme immunoassay detection using a commercial kit for diagnosis of Norovirus, what can the assay detect?
Either
1) Virus in clinical sample using abs to capsid antigens
• sensitivity:104-106 viral particles/ml
2) IgG antibodies to NVs in patients sera using viral antigen (capsid proteins)
• test by day 5 of infection and then between the 3rd and 6th week to test for seroconversion)
